Foodborne salmonellosis is a major public health problem. Salmonella is the second leading cause of annual foodborne illness cases, with an estimated 1,340,000 cases compared with nearly 2,000,000 for Campylobacter. A survey conducted in the United States estimated that in 1999 non-typhoidal salmonellosis of foodborne origin caused approximately 15,600 hospitalizations and 550 deaths (Mead, P. S., L. Slutsker, V. Dietz, et al. Food-related illness and death in the United States. Emerg. Infect. Dis. 5:607-625. 1999). Although the genus Salmonella has more than 2300 serovars, only a relatively restricted number belonging mainly to the enterica subspecies of the Salmonella enterica species are responsible for the great majority of human infections. The most common sources of Salmonella infections in humans are contaminated foods, including eggs, poultry, produce, meat, and meat products. Eggs and poultry meat are recognized as the major vehicles of human infections because of epizootics in fowl.
Early detection of foodborne Salmonella is vital for food safety assurance. However, conventional methods for detecting foodborne Salmonella are laborious and time consuming. These methods typically involve identifying presumptively positive samples by sequentially processing samples in a pre-enrichment phase, a selective-enrichment phase, and then an analysis phase. The analysis phase may involve culturing the enriched sample on selective differential agar, analyzing with polymerase chain reaction (PCR), and/or analyzing via immunoassay. After identifying the presumptively positive samples, confirmation of the presumptively positive samples typically requires biochemical characterization of isolates obtained from selective, differential agar media. See, e.g., U.S. Food and Drug Administration (2011) Bacteriological Analytical Manual, Chapter 5. It is estimated that millions of such Salmonella analyses are run routinely in the United States each year.
Early detection of Salmonella is needed if foodborne illnesses caused by Salmonella are to be reduced. While determinative microbiology requires confirmation of presumptively positive samples, this is not the case in many applications such as environmental monitoring and food safety-HACCP testing. In such applications, only a reasonable presumption that a sample is contaminated is required to take such corrective actions as modifying a sanitation procedure or quarantining a product lot pending subsequent confirmation. Raid screening tests which have comparable diagnostic performance to culture methods or other rapid molecular methods can suffice.
An example of a rapid screening test for Listeria is described in U.S. Pat. No. 7,960,164. The Listeria screening test employs a selective Listeria indicator broth, which both enriches and indicates the presence of any Listeria within the sample. This test permits identification of presumptively positive samples within 24 to 48 hours while simultaneously identifying negative samples. The presumptively positive samples can be confirmed by submitting them to a reference or central laboratory. The system saves time and money by not requiring any further processing of the negative samples. Further, the early detection of presumptively positive samples can provide useful information about the status of critical control points, particularly sanitized working surfaces.
Several Salmonella-selective enrichment media and systems are known in the art. See, e.g., U.S. Pat. No. 4,279,995 to Woods et al.; U.S. Pat. No. 5,208,150 to Tate et al.; U.S. Food and Drug Administration (2011) Bacteriological Analytical Manual, Chapter 5; U.S. Pat. No. 7,704,706 to Druggan; U.S. Pat. No. 7,150,977 to Restaino; U.S. Pat. No. 6,368,817 to Perry et al.; U.S. Pat. No. 5,434,056 to Monget et al.; and U.S. Pat. No. 5,194,374 to Rambach. The media and systems described in these references do not provide sufficient selectivity of Salmonella spp.
There exists a need for a simple, rapid screening test that identifies presumptively positive Salmonella samples at an early stage of the sample analysis, preferably in a selective enrichment stage of the analysis. Simultaneous enrichment and detection of Salmonella using a single testing method would reduce not only time but also the cost of labor and media. Streamlining procedures and reducing labor and test costs should permit more frequent monitoring for Salmonella, thereby reducing contamination hazard.